JP2008069044A - Ceramic substrate, ceramic circuit board using the same and semiconductor module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ceramic substrate in which crack is hardly caused or developed in a heating-cooling cycle and having excellent durability, a ceramic circuit board using the same and a semiconductor module. <P>SOLUTION: The ceramic substrate has ≥6.0 MPa m<SP>1/2</SP>fracture toughness and ≥230 GPa bending modulus. The bending modulus is controlled to ≥230 GPa by controlling the Picker's hardness of the surface of the ceramic substrate to ≥1600. Otherwise, the bending modulus is controlled to ≥ 230 GPa by controlling residual stress of the surface of the ceramic substrate to ≤-40 MPa. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a ceramic substrate having high strength and high electrical insulation and high thermal conductivity. The present invention also relates to a ceramic circuit board and a semiconductor module using the ceramic substrate. In particular, it is a technology that can be effectively applied to a power semiconductor module that requires high reliability with respect to a cooling cycle.

  In recent years, power semiconductor modules (IGBT, power MOSFET, etc.) capable of high voltage and large current operation are used as inverters for electric vehicles. As a substrate used for a power semiconductor module, a conductive metal plate (metal circuit plate) that becomes a circuit is joined to one surface (upper surface) of an insulating ceramic substrate made of aluminum nitride or silicon nitride, and the other surface. Ceramic circuit boards in which a metal plate for heat dissipation (metal heat dissipation plate) is bonded to the (lower surface) are widely used. As this metal plate, a copper plate or an aluminum plate is used. A semiconductor element or the like is mounted on the upper surface of the metal circuit board. For joining the ceramic substrate and the metal plate, an active metal method using a brazing material or a so-called copper direct joining method in which a copper plate is directly joined is employed.

However, in a power semiconductor module using a ceramic circuit board in which a metal circuit board and a metal heat sink are bonded to a ceramic board, the thickness of the metal circuit board and the metal heat sink is compared with 0.5 mm or more so that a large current can flow. May be thicker. At this time, in particular, when copper or copper alloy having high thermal conductivity is used for the metal circuit board and the metal heat sink, the thermal expansion coefficient of the insulating ceramic and the copper are greatly different (for example, the silicon nitride ceramic is 3.0 × 10 ^It is about ⁻⁶ / K, and copper is 16.7 × 10 ⁻⁶ / K). For this reason, a great amount of thermal stress is generated in the cooling process after joining and the cooling cycle during operation of the power semiconductor module. This stress is present as compressive and tensile residual stress in the vicinity of the joint portion of the ceramic substrate. This residual stress causes cracks in the ceramic substrate, causes a breakdown voltage failure, or causes peeling of the metal circuit board and the metal heat sink.

  In practice, in order to construct a power semiconductor module using such a ceramic circuit board, a semiconductor element is mounted on the metal circuit board by soldering. Furthermore, depending on the structure of the power semiconductor module, the metal heat sink is joined to the metal base plate by soldering. Thus, it is necessary to go through a heating and cooling process during mounting. In addition, such ceramic circuit boards are required to have reliability that ensures heat dissipation characteristics even after a predetermined number of heating and cooling cycle tests from −55 ° C. to 150 ° C., for example. Depending on the field of use, for example, it is required to pass a heating / cooling cycle test of 200 times or more, 1000 times or more, and further 3000 times or more. In particular, when mounted on a hybrid vehicle, an electric vehicle, a train, an aircraft, or the like, high reliability with high durability is required.

  In this respect, the aluminum nitride substrate has high thermal conductivity, but its mechanical strength and fracture toughness are low, so it is not highly reliable as a ceramic substrate. On the other hand, since the silicon nitride substrate has a relatively high thermal conductivity and high mechanical properties as a ceramic substrate, it is considered that a highly reliable ceramic circuit substrate can be realized.

  On the other hand, although ceramic substrates having relatively high thermal conductivity such as aluminum nitride substrates and silicon nitride substrates have been developed in recent years, ceramic substrates have a lower thermal conductivity than metal plates. This is a factor that hinders heat dissipation when configured. Therefore, in order to reduce the thermal resistance of the power semiconductor module, it is necessary to increase the thickness of the metal plate having a high thermal conductivity and to reduce the thickness of the ceramic substrate having a lower thermal conductivity than the metal plate.

  However, if the ceramic substrate is made thin, it will break at low stress, so even if a silicon nitride substrate with high mechanical properties is used for the ceramic substrate, cracks may occur due to thermal stress from the heating / cooling cycle test. Became high.

  This crack often occurs at the outer peripheral portion of the pattern of the metal circuit board, particularly at the corner portion, which deteriorates the dielectric strength and strength of the silicon nitride substrate, and when a voltage is applied to the mounted semiconductor element, the silicon nitride substrate There was also a breakdown. Therefore, the reliability of a semiconductor module in which a semiconductor element is mounted on such a ceramic circuit board has not been sufficient.

As a technology in a ceramic circuit board for improving the reliability with respect to such a heating / cooling cycle, in Patent Document 1, the three-point bending strength of a silicon nitride substrate is 500 MPa or more, and the fracture toughness value is 6.5 MPa · m ^1/2. The technology described above is described. By using a silicon nitride substrate having a three-point bending strength of 500 MPa or more and a high fracture toughness value of 6.5 MPa · m ^1/2 or more, generation of cracks due to thermal stress is suppressed. Hereinafter, this technique is referred to as a first conventional example.

  Patent Document 2 describes a technique using a silicon nitride substrate having a low Young's modulus. The reliability of the ceramic substrate is improved by lowering the Young's modulus and making the ceramic substrate easier to bend. Hereinafter, this technique is referred to as a second conventional example.

  Patent Document 3 describes that the surface roughness of a ceramic substrate is given directionality. By giving the direction to the surface roughness of the ceramic substrate by grinding the surface, the bending strength is improved and the reliability of the ceramic substrate is improved. Hereinafter, this technique is referred to as a third conventional example.

JP 2002-201075 A JP 2005-101624 A JP 2001-181054 A

  In the ceramic circuit board according to the first conventional example described above, the fracture toughness of the ceramic board is increased. However, in order to ensure the reliability of the ceramic circuit board, it is necessary to increase the fracture toughness in order to suppress the propagation of cracks inside the ceramic substrate and at the same time suppress the occurrence of cracks from the surface of the ceramic substrate. Further, in the first conventional example, the use of a ceramic substrate having a high bending strength is also described, but even if the bending strength is increased, it does not necessarily lead to the suppression of the occurrence of cracks from the surface of the ceramic substrate.

  In the ceramic circuit board according to the second conventional example described above, the ceramic substrate is easily bent by reducing the Young's modulus of the ceramic substrate measured by the ultrasonic pulse method, but as in the first conventional example. The suppression of the occurrence of cracks from the ceramic substrate surface is not considered.

  In this regard, in the above-described third conventional example, the occurrence of cracks from the surface of the ceramic substrate is suppressed by providing directionality to the surface roughness of the surface of the ceramic substrate. However, there is a problem that a process such as polishing with a rotating grindstone needs to be applied, and the processing is costly.

  The present invention has been made in view of such problems, and an object thereof is to provide an invention that solves the above problems.

In order to solve the above problems, the present invention has the following configurations.
The gist of the invention described in claim 1 resides in a ceramic substrate having a fracture toughness of 6.0 MPa · m ^1/2 or more and a flexural modulus of 230 GPa or more.

  The gist of the invention described in claim 2 resides in the ceramic substrate according to claim 1, wherein the surface has a Vickers hardness of 1600 or more.

  The gist of the invention described in claim 3 resides in the ceramic substrate according to claim 1 or 2, wherein the residual stress on the surface is -40 MPa or less.

  The gist of the invention described in claim 4 resides in the ceramic substrate according to any one of claims 1 to 3, wherein the ceramic substrate is a silicon nitride ceramic substrate.

  The gist of the invention described in claim 5 resides in the ceramic substrate according to claim 4, wherein the degree of orientation of the surface is 0.15 or more.

  The gist of the invention described in claim 6 resides in the ceramic substrate according to any one of claims 1 to 5, wherein the thickness is 0.1 to 0.3 mm.

  The gist of the invention described in claim 7 is that the ceramic substrate according to any one of claims 1 to 6, a metal circuit board bonded to one surface of the ceramic substrate, and another surface of the ceramic substrate. It exists in the ceramic circuit board characterized by consisting of the joined metal heat sink.

  The gist of the invention described in claim 8 resides in the ceramic circuit board according to claim 7, wherein the metal circuit board is a copper plate or a copper alloy plate having a thickness of 0.5 to 3.0 mm.

  The gist of the invention described in claim 9 resides in a semiconductor module comprising the ceramic circuit board according to claim 7 or 8 and a semiconductor element mounted on the ceramic circuit board.

  Since the present invention is configured as described above, it is possible to obtain a highly reliable ceramic circuit board and semiconductor module that are less likely to crack in the ceramic substrate during heating and cooling cycles during manufacturing and driving. it can.

In the ceramic substrate according to the embodiment of the present invention, the fracture toughness is 6.0 MPa · m ^1/2 or more, and the flexural modulus is 230 GPa or more. Here, the fracture toughness was measured by the IF method based on JIS R1607. A Vickers indenter was pressed into the cross section of the mirror-polished ceramic substrate with a load of 2 kgf, and the fracture toughness value was calculated from the diagonal length of the indentation and the crack propagation length. The flexural modulus was measured by a static elastic modulus test method based on JIS R1602. After processing the ceramic substrate into a test piece having a width of 4 mm and setting it on a three-point bending jig with a distance between support rolls of 7 mm, a crosshead speed of 0.5 mm / min. The load was applied at, and the load applied to the test piece and the displacement of the test piece at the load point were calculated.

  When the ceramic substrate is destroyed by the heating / cooling cycle of the ceramic circuit substrate and the semiconductor module, the ceramic substrate is cracked near the surface of the outer periphery of the pattern of the metal circuit board, and then propagates horizontally in the ceramic substrate There are many. Therefore, a highly reliable ceramic circuit board and semiconductor module can be obtained by suppressing the occurrence of cracks on the surface of the ceramic substrate and the development of cracks inside the ceramic substrate.

The suppression of crack propagation inside the ceramic substrate can be dealt with by increasing the fracture toughness of the ceramic substrate. The fracture toughness of a ceramic substrate is a value that indicates the difficulty of crack growth. By setting the fracture toughness to 6.0 MPa · m ^1/2 or more, the heating and cooling cycle of the ceramic circuit board and semiconductor module can be applied to the interior of the ceramic substrate. It is possible to suppress the development of cracks. There are various methods for increasing the fracture toughness of a ceramic substrate. For a silicon nitride ceramic substrate, for example, a sintering aid containing a rare earth oxide is added to a silicon nitride raw material powder having a BET specific surface area of 15 m ² / g or less. In addition, by sintering at a high temperature of 1800 ° C. or higher in a nitrogen atmosphere, a crystal structure entangled with columnar particles having a strong grain boundary phase can be obtained, and the fracture toughness is 6.0 MPa · m due to high crack propagation resistance. It can be increased to ^1/2 or more.

On the other hand, in order to suppress the generation of cracks on the surface of the ceramic substrate, the inventors have found that it is important to increase the bending elastic modulus of the ceramic substrate to 230 GPa or higher. In order to suppress the occurrence of cracks on the surface of the ceramic substrate, it is necessary to increase the rigidity of the surface of the ceramic substrate. Therefore, generation of cracks can be suppressed by increasing the elastic modulus of the ceramic substrate surface. The bending elastic modulus of the ceramic substrate is determined by the elastic modulus of the ceramic substrate surface and inside, but when the fracture toughness is as high as 6.0 MPa · m ^1/2 , the elastic modulus inside the ceramic substrate is low. Is susceptible to the elastic modulus of the ceramic substrate surface. That is, by setting the fracture toughness to 6.0 MPa · m ^1/2 or more and the bending elastic modulus to 230 GPa or more, the bending elastic modulus of the ceramic substrate is increased, and the generation of cracks from the surface of the ceramic substrate can be suppressed. . When the flexural modulus is greater than 300 GPa, a large elastic strain energy is accumulated in the ceramic substrate due to the heating / cooling cycle of the ceramic circuit substrate and the semiconductor module, and there is a possibility of breaking at a time when a crack occurs. Therefore, the bending elastic modulus of the ceramic substrate is preferably in the range of 230 to 300 GPa.

As a result of intensive studies by the present inventors, when the fracture toughness of the ceramic substrate is 6.0 MPa · m ^1/2 or more and the flexural modulus is 230 GPa or more, the ceramic circuit board and the semiconductor module are subjected to heating and cooling cycles. Cracks were generated in the substrate and progressed without causing destruction.

  In an actual ceramic substrate, by setting the surface Vickers hardness to 1600 or more, the flexural modulus was 230 GPa or more, and the generation of cracks on the ceramic substrate surface could be suppressed. Here, the Vickers hardness was measured according to JIS R1610. The upper limit of the surface Vickers hardness is preferably about 1800. Exceeding this is not preferable because the flexural modulus of the ceramic substrate exceeds 300 GPa.

  Furthermore, the generation of cracks on the surface of the ceramic substrate was suppressed by applying compressive residual stress to the surface of the ceramic substrate. By setting the residual stress on the surface to −40 MPa or less (40 MPa or more in absolute value), the flexural modulus was 230 GPa or more. Here, the residual stress on the surface of the ceramic substrate was measured by an X-ray stress measurement method that measures the surface stress using X-ray diffraction. Here, the negative (-) symbol of the residual stress indicates a compressive stress.

  Further, when the ceramic substrate is made of silicon nitride ceramics mainly composed of anisotropic crystal particles, the bending elastic modulus is 230 GPa or more by setting the orientation degree of the surface of the silicon nitride ceramic substrate to 0.15 or more. It became. The upper limit of the degree of orientation is preferably about 0.45. Exceeding this is not preferable because the flexural modulus of the ceramic substrate exceeds 300 GPa. Here, the degree of orientation of the surface of the silicon nitride ceramic substrate is the thickness of the silicon nitride ceramic substrate determined from the ratio of the X-ray diffraction line intensity of each predetermined lattice plane of β-type silicon nitride particles constituting the silicon nitride ceramic substrate. This indicates the ratio of orientation in a plane perpendicular to the direction, and the degree of orientation is an expression given by FK Lotgerling based on the X-ray diffraction lines generated by irradiating the surface of the silicon nitride ceramic substrate with X-rays ( 1) (see FK Lotgerling: J. Inorg, Nucl. Chem., 9 (1959) 113.).

(Orientation degree) = (P−P ₀ ) / (1−P ₀ ) (1)
In the formula (1), P is represented by the formula (2), and the (110) plane, the (200) plane, the (210) plane, the (310) plane of the β-type silicon nitride particles on the surface of the silicon nitride ceramic substrate, and This means the ratio of the intensity of each X-ray diffraction line on the (320) plane, and P ₀ is expressed by the formula (3), and the (110) plane of the β-type silicon nitride particles in the β-type silicon nitride powder, (200) It means the ratio of the X-ray diffraction line intensity of each of the plane, (210) plane, (310) plane and (320) plane.
P = (I ₍₁₁₀₎ + I ₍₂₀₀₎ + I ₍₂₁₀₎ + I ₍₃₁₀₎ + I ₍₃₂₀₎ ) / (I ₍₁₁₀₎ + I ₍₂₀₀₎ + I ₍₁₀₁₎ + I ₍₂₁₀₎ + I ₍₂₀₁₎ + I ₍₃₁₀₎ + I ₍₃₂₀₎ + I ₍₀₀₂₎ ) (2)
P ₀ = (I ′ ₍₁₁₀₎ + I ′ ₍₂₀₀₎ + I ′ ₍₂₁₀₎ + I ′ ₍₃₁₀₎ + I ′ ₍₃₂₀₎ ) / (I ′ ₍₁₁₀₎ + I ′ ₍₂₀₀₎ + I ′ ₍₁₀₁₎ + I ′ _{( 210)} + I ′ ₍₂₀₁₎ + I ′ ₍₃₁₀₎ + I ′ ₍₃₂₀₎ + I ′ ₍₀₀₂₎ ) (3)

The ceramic substrate preferably has a thickness of 0.1 to 0.4 mm. When the ceramic substrate is thicker than 0.4 mm, when the ceramic circuit substrate and the semiconductor module are configured, the portion of the ceramic substrate having a relatively low thermal conductivity becomes thick, so that the heat dissipation becomes worse. Moreover, even if the bending elastic modulus is lower than 230 GPa, it is difficult to bend, and therefore cracks are unlikely to occur. On the other hand, when the ceramic substrate is thinner than 0.1 mm, the heat dissipation is improved, but it is easily bent due to the thermal stress generated by the thermal expansion difference between the metal circuit board and the metal heat sink. It is difficult to suppress the occurrence of cracks. Therefore, when a ceramic substrate having a fracture toughness of 6.0 MPa · m ^1/2 or more and a flexural modulus of 230 to 300 GPa has a thickness of 0.1 to 0.4 mm, a particularly reliable ceramic substrate is obtained.

In the ceramic circuit board comprising a ceramic substrate, a metal circuit board bonded to one surface of the ceramic substrate, and a metal heat sink bonded to the other surface of the ceramic substrate, the metal circuit plate and the metal heat sink are Each is preferably a copper plate or a copper alloy plate of 0.5 to 5.0 mm. By making the metal circuit board and the metal heat radiating plate a copper plate or a copper alloy plate having a high thermal conductivity of 0.5 mm or more in thickness, a ceramic circuit board with good heat dissipation can be provided and the thickness can be reduced. Ceramics with a thickness of 5.0 mm or less can reduce thermal stress due to the difference in thermal expansion between the ceramic substrate, the metal circuit board, and the metal heat sink, a fracture toughness of 6.0 MPa · m ^1/2 or more, and a flexural modulus of 230 GPa or more. With the substrate, a highly reliable ceramic circuit substrate can be obtained.

Thus, a ceramic circuit board with high reliability can be obtained by setting the fracture toughness to 6.0 MPa · m ^1/2 or more and the bending elastic modulus to 230 GPa or more in the ceramic substrate. Therefore, the reliability with respect to the heating / cooling cycle of the semiconductor module configured by mounting the semiconductor element on the ceramic circuit board is also increased.

  Examples of the present invention will be described below. However, the present invention is not limited to these examples.

A silicon nitride ceramic substrate was produced as the ceramic substrate. A ball mill using a silicon nitride raw material powder having a BET specific surface area of 3 to 11 m ² / g with a predetermined amount of a sintering aid composed of Y ₂ O ₃ and MgO and using silicon nitride balls as a grinding medium in an organic solvent Wet mixed by. An organic binder, a plasticizer, and the like were mixed into this mixture and uniformly mixed with a ball mill to obtain a raw material slurry. The raw slurry was defoamed and thickened, and then formed into a predetermined plate thickness by a conventionally known doctor blade method to obtain a molded body. This sheet molded body was cut into a predetermined shape, degreased at 500 ° C., and further sintered in a sintering furnace at a temperature of 1800 to 1900 ° C. for 2 to 5 hours in a nitrogen pressurized atmosphere of 0.9 MPa. The obtained sintered body was heat-treated and blasted to produce a silicon nitride ceramic substrate.

  Table 1 shows the thickness, fracture toughness, bending strength, and flexural modulus of the silicon nitride ceramic substrate. The bending strength was measured by a three-point bending test according to JIS R1601. After processing the ceramic substrate into a test piece having a width of 4 mm and setting it on a three-point bending jig with a distance between support rolls of 7 mm, a crosshead speed of 0.5 mm / min. The load was applied at, and calculated from the load applied to the test piece at the time of breakage.

  A ceramic circuit board was produced using the produced silicon nitride ceramic board and a copper plate as the metal circuit board and the metal heat sink. An active metal brazing material was printed on both sides of the silicon nitride ceramic substrate, and a rectangular copper plate substantially the same as the silicon nitride ceramic substrate was heated and bonded to both sides. After cooling, the metal circuit board and the metal heat sink were etched so as to have a predetermined pattern, and Ni-P plating was applied to the metal circuit board and the metal heat sink to produce a silicon nitride ceramic circuit board.

  The obtained silicon nitride ceramic circuit board was subjected to a thermal shock test as shown below. The thermal shock test was held at 300 ° C. for 10 minutes and then rapidly cooled to room temperature to examine whether or not the silicon nitride ceramic substrate was cracked. This operation was repeated, and the number of thermal shock tests until a crack occurred was evaluated. Table 1 shows the number of thermal shock tests in which cracks occurred in the silicon nitride ceramic circuit board. The thermal shock test is performed up to 15 times, and when no crack is generated, it is described as ≧ 15.

  A semiconductor element was soldered onto a metal circuit board of a silicon nitride ceramic circuit board, and then wire bonding was performed to obtain a semiconductor module. The obtained semiconductor module was placed on a water-cooled copper jacket set at 20 ° C. via a high thermal conductive grease, a current 14A was applied to the semiconductor module, and the change in voltage applied to the semiconductor element after 1 second was measured. The thermal resistance was measured by determining the increase value of the element temperature from the relationship between the temperature and voltage of the semiconductor element that had been measured in advance. Table 1 shows the thermal resistance value of the semiconductor module.

When a silicon nitride ceramic substrate having a thickness of 0.32 mm with a fracture toughness of 6 MPa · m ^1/2 or more and a flexural modulus of 230 GPa or more is employed as in Examples 1 to 12, the metal circuit board has a thickness of 0. In a silicon nitride ceramic circuit board in which an 8 mm copper plate and a metal heat radiating plate are 0.6 mm thick copper plates, the number of thermal shock tests until a crack occurred was 10 times or more.

When the fracture toughness of the silicon nitride ceramic substrate is lower than 6 MPa · m ^1/2 as in Comparative Examples 1 to 9, when the flexural modulus is lower than 230 GPa, or when both the fracture toughness and the flexural modulus are low, the silicon nitride ceramics In the circuit board, a crack was generated by a thermal shock test less than 10 times.

  Especially in Comparative Examples 5-7, although bending strength was as high as 750 Mpa or more, since the bending elastic modulus was lower than 230 GPa, the crack was generated in the silicon nitride ceramic circuit board by the thermal shock test less than 10 times.

Further, the metal circuit board and the metal heat radiating plate were a copper plate having a thickness of 1.5 mm in Example 13, and a copper plate having a thickness of 2.5 mm in Example 14. Even when a relatively thick copper plate is joined as in Examples 13 and 14, silicon nitride ceramics are obtained by using a silicon nitride ceramic substrate having a fracture toughness of 6 MPa · m ^1/2 or more and a flexural modulus of 230 GPa or more. In the circuit board, the number of times of the thermal shock test until the crack occurred was 10 times or more.

In Examples 15 to 20, the thickness of the silicon nitride ceramic substrate was set to 0.2 mm or 0.15 mm. Even if the thickness of the silicon nitride ceramic substrate is relatively thin, the use of the silicon nitride ceramic substrate having a fracture toughness of 6 MPa · m ^1/2 or more and a flexural modulus of 230 GPa or more causes cracks in the silicon nitride ceramic circuit substrate. The number of thermal shock tests before the occurrence of was 10 or more.

In particular, in Examples 15, 17, 19 and 20, the bending strength of the silicon nitride ceramic substrate was relatively low at 750 MPa or less, but the fracture toughness was 6 MPa · m ^1/2 or more and the bending elastic modulus was 230 GPa or more. In the silicon nitride ceramic circuit board, the number of thermal shock tests until a crack occurred was 10 times or more.

  As in Examples 15 to 20, in the ceramic circuit substrate and the semiconductor module, reducing the thickness of the ceramic substrate improves heat dissipation. In Examples 15 to 20, although the number of thermal shock tests is ≧ 15, the thermal resistance of the semiconductor module shows a relatively low value, and a highly reliable silicon nitride ceramic circuit board excellent in heat dissipation is obtained. It was.

  In Comparative Example 10, since the silicon nitride ceramic substrate is as thick as 0.54 mm, it is difficult to elastically deform. Therefore, even if the flexural modulus was as low as 171 GPa, the number of thermal shock tests until the cracks occurred in the silicon nitride ceramic circuit board was 10 times or more. However, since the silicon nitride ceramic substrate is relatively thick, the thermal resistance of the semiconductor module also shows a relatively high value, resulting in a silicon nitride ceramic circuit substrate inferior in heat dissipation.

Next, a ceramic substrate in which the surface Vickers hardness is changed will be described. Silicon nitride ceramic substrates having different surface Vickers hardnesses were produced by changing the amount of sintering aid added and the heat treatment temperature when producing the ceramic substrate. In the method for producing a silicon nitride ceramic substrate, a silicon nitride raw material powder having a BET specific surface area of 6 m ² / g and a sintering aid of 4.0 to 5.5 wt% are used as the raw material powder. The test was carried out at a temperature of 0 ° C. for 5 hours in a nitrogen pressurized atmosphere of 0.9 MPa. The obtained sintered body was subjected to blasting after heat treatment or only blasting to produce a silicon nitride ceramic substrate. The blast treatment was performed by wet-spraying alumina abrasive grains having an average particle diameter of 45 μm on both surfaces of the silicon nitride ceramic substrate at a pressure of 0.2 MPa. The heat treatment was performed at a temperature of 1450 to 1800 ° C. for 5 hours. The atmosphere during the heat treatment was a 0.9 MPa nitrogen pressure atmosphere in order to suppress decomposition of silicon nitride under the heat treatment temperature of 1800 ° C., and a nitrogen atmospheric pressure atmosphere under other conditions.

  Using the obtained silicon nitride ceramic substrate having a thickness of 0.32 mm and a copper plate having a thickness of 0.8 mm and 0.6 mm as a metal circuit plate and a metal heat sink, a ceramic circuit substrate was produced. Table 2 shows the flexural modulus, the surface Vickers hardness, and the number of thermal shock tests until cracks are generated, along with the amount of sintering aid added and heat treatment conditions. When the Vickers hardness of the surface of the ceramic substrate is 1600 or more as in Examples 1 to 7, the flexural modulus is 230 GPa or more, and the number of thermal shock tests until the crack occurs in the ceramic circuit substrate is 10 times or more. there were. Preferred production conditions for imparting a high flexural modulus are a sintering aid addition amount of 4.5 wt% or more and a heat treatment temperature of 1550 ° C. or more.

Next, a ceramic substrate in which the residual stress on the surface is changed will be described. Since the sintered ceramic substrate has a large surface roughness, blasting is performed to reduce the surface roughness by spraying abrasive grains. By changing the pressure for spraying the abrasive grains in the blasting process, ceramic substrates having different surface residual stresses were produced. In the method for producing a silicon nitride ceramic substrate, a silicon nitride raw material powder having a BET specific surface area of 6 m ² / g and a sintering aid of 5.0 wt% are used as the raw material powder, and sintering is performed at a temperature of 1850 ° C. The test was carried out in a nitrogen pressure atmosphere of 0.9 MPa for 5 hours. The obtained sintered body was heat treated in a nitrogen pressurized atmosphere of 0.9 MPa at a temperature of 1800 ° C. for 5 hours, and then blasted to prepare a silicon nitride ceramic substrate. The blast treatment was performed on both surfaces of the silicon nitride ceramic substrate by wet jetting alumina abrasive grains having an average particle size of 45 μm at a pressure of 0.05 to 0.5 MPa. Note that if the heat treatment is performed after the blast treatment first, the compressive stress applied by the blast treatment is alleviated by the heat treatment.

  Using the obtained silicon nitride ceramic substrate having a thickness of 0.32 mm and a copper plate having a thickness of 0.8 mm and 0.6 mm as a metal circuit plate and a metal heat sink, a ceramic circuit substrate was produced. Table 3 shows the flexural modulus of the employed silicon nitride ceramic substrate, the residual stress on the surface, and the number of thermal shock tests until cracks are generated, together with the blasting conditions. When the surface of the ceramic substrate has a compressive residual stress of −40 MPa or less as in Examples 8 to 10, the flexural modulus is 230 GPa or more, and the number of thermal shock tests until a crack occurs in the ceramic circuit substrate is 10 times or more.

Next, a silicon nitride ceramic substrate having a different degree of surface orientation will be described. Silicon nitride ceramic substrates having different degrees of surface orientation were produced by changing the silicon nitride powder as the main raw material of the silicon nitride ceramic substrate and the sintering conditions. In the manufacturing method of the silicon nitride ceramic substrate, raw material powder A, B, C, BET specific surface area of each as ^{D 5.8m 2 /g,3.5m 2 /g,5.4m 2 /} g, 11m 2 / The sintering was carried out at a temperature of 1850 ° C. or 1900 ° C. for 5 hours in a nitrogen pressure atmosphere of 0.9 MPa using g of each silicon nitride powder and 5.0 wt% sintering aid. The obtained sintered body was heat treated in a nitrogen pressurized atmosphere of 0.9 MPa at a temperature of 1800 ° C. for 5 hours and then blasted. The blast treatment was performed by wet-spraying alumina abrasive grains having an average particle diameter of 45 μm on both surfaces of the silicon nitride ceramic substrate at a pressure of 0.2 MPa.

  Using the obtained silicon nitride ceramic substrate having a thickness of 0.32 mm and a copper plate having a thickness of 0.8 mm and 0.6 mm as a metal circuit plate and a metal heat sink, a ceramic circuit substrate was produced. Table 4 shows the flexural modulus of the employed silicon nitride ceramic substrate, the degree of orientation of the surface, and the number of thermal shock tests until cracks are generated, together with the type of silicon nitride raw material powder and the sintering conditions. When the degree of orientation of the surface of the silicon nitride ceramic substrate is 0.15 or more as in Examples 2, 11 and 12, the flexural modulus is 230 GPa or more, and the thermal shock test until cracks occur in the ceramic circuit substrate The number of times was 10 times or more.

  As described above, by adjusting the Vickers hardness, residual stress, or orientation degree, and obtaining a ceramic substrate having a flexural modulus of 230 GPa or more, a ceramic circuit substrate in which the generation of cracks in the heating and cooling process is suppressed can be obtained.

  The semiconductor element was solder-bonded on the metal circuit boards of the silicon nitride ceramic circuit boards of Examples 1 to 20 and Comparative Examples 1 to 10, and then wire bonding was performed to obtain a semiconductor module. About this semiconductor module, the heat cycle test was done as shown below. In the heat cycle test, a temperature rising / falling cycle in which cooling at −55 ° C. for 20 minutes, holding at room temperature for 10 minutes and heating at 150 ° C. for 20 minutes is defined as one cycle, and this is repeated 3000 cycles. The progress of cracks in the ceramic substrate was investigated. As a result, in Examples 1 to 20, it was found that there was no semiconductor module in which the silicon nitride ceramic substrate was destroyed due to the progress of cracks, and the semiconductor module had a high cycle life with respect to the heating and cooling cycle. In the semiconductor modules of Comparative Examples 1 to 9, the development of cracks was observed from the vicinity of the surface of the silicon nitride ceramic substrate at the outer periphery of the pattern of the metal circuit board after the heat cycle test. In the semiconductor module of Comparative Example 10, the silicon nitride ceramic substrate did not break due to the progress of cracks, but as shown in Table 1, the thermal resistance was relatively high and the heat dissipation was poor.

The ceramic substrate of the present invention can be effectively applied to a power semiconductor module that requires high reliability with respect to a cooling cycle.

Claims (9)

A ceramic substrate having a fracture toughness of 6.0 MPa · m ^1/2 or more and a flexural modulus of 230 GPa or more.   The ceramic substrate according to claim 1, wherein the surface has a Vickers hardness of 1600 or more.   The ceramic substrate according to claim 1 or 2, wherein the surface has a residual stress of -40 MPa or less.   The ceramic substrate according to any one of claims 1 to 3, wherein the ceramic substrate is a silicon nitride ceramic substrate.   The ceramic substrate according to claim 4, wherein the degree of orientation of the surface is 0.15 or more.   The ceramic substrate according to any one of claims 1 to 5, wherein the thickness is 0.1 to 0.4 mm.   It consists of the ceramic substrate of any one of Claims 1 thru | or 6, the metal circuit board joined to one surface of the ceramic substrate, and the metal heat sink joined to the other surface of the ceramic substrate. A characteristic ceramic circuit board.   The ceramic circuit board according to claim 7, wherein the metal circuit board and the metal heat radiating board are each a copper plate or a copper alloy plate having a thickness of 0.5 to 5.0 mm. A semiconductor module comprising the ceramic circuit board according to claim 7 or 8 and a semiconductor element mounted on the ceramic circuit board.